Bilayer passivation structure for photovoltaic cells

ABSTRACT

An improved photovoltaic cell, according to one embodiment, includes a base layer; a primary window layer having a first type of doping, with the primary window layer being disposed over the base layer; and a secondary window layer having the first type of doping, with the secondary window layer being disposed over the primary window layer. In another embodiment, the improved photovoltaic cell has a multilayer back-surface field structure; a base layer disposed over the back-surface field structure; and a primary window layer disposed over the base layer. In yet another embodiment, the photovoltaic cell includes a base layer; and a primary window layer disposed over the base layer, with the primary window layer having a thickness of at least about 1000 Angstroms.

This invention was made with Government support under contract numberF33615-95-C5561 awarded by the Government. The Government has certainrights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to photovoltaic cells and, moreparticularly, to an improved photovoltaic cell having a passivationstructure that results in improved performance and efficiency.

2. Description of Related Art

The interest in photovoltaic (PV) cells continues as concerns overpollution and limited resources continue. The continued interest hasbeen in both terrestrial and non-terrestrial applications. In thenon-terrestrial environment of outer space, the concern over limitedresources of any type is a major one. This is because the need toincrease the amount of a resource increases the payload. And anincreased payload can increase the cost of a launch more than linearly.But with the ready availability of solar energy in outer space for aspacecraft such as a satellite, the conversion of solar energy intoelectrical energy is an obvious alternative to increased payload.Irrespective of the application, and as with any energy generationsystem, efforts have been ongoing into increasing the output and/orefficiency of PV cells. In terms of output, multiple cells or layershaving different energy bandgaps have been stacked so that each cell orlayer can absorb a different part of the wide energy distribution in thesunlight. The stacked arrangement has been provided in a monolithicstructure on a single substrate or on multiple substrates. Examples ofmulti-cell devices are shown in U.S. Pat. Nos. 5,800,630; 5,407,491;5,100,478; 4,332,974; 4,255,211; and 4,017,332.

In the multiple cell device, semiconductor materials are typicallylattice matched to form multiple p-n (or n-p) junctions. The p-n (orn-p) junctions can be of the homojunction or heterojunction type. Whensolar electromagnetic energy is received at a junction, excess chargecarriers (i.e., electrons and holes) are generated in the conduction andvalence bands in the semiconductor materials adjacent the junction. Avoltage is thereby created across the junction and a current can beutilized therefrom. As the solar electromagnetic energy passes to thenext junction which has been optimized to a lower energy range,additional solar energy but at this lower energy range can be convertedinto a useful current. With a greater number of junctions, there can begreater conversion efficiency and increased output voltage.

But for the multiple cell PV device, efficiency is limited by therequirement of low resistance interfaces between the individual cells toenable the generated current to flow from one cell to the next.Accordingly, in a monolithic structure, tunnel junctions and otherconductive interfacing layers have been used to minimize the blockage ofcurrent flow. In a multiple wafer structure, metal grids or transparentconductive layers have been used for low resistance connectivity.

Another limitation to the multiple cell PV device is that current outputat each junction must be the same for optimum efficiency in the seriesconnection. Also, there is a practical limit on the number of junctions,since each successive junction incurs losses compared to a theoreticalmaximum.

The concern over efficiency in PV cells has created more interest in theuse of germanium, gallium arsenide, indium phosphide, and gallium indiumphosphide, which tend to be more efficient than their siliconpredecessor. Indium phosphide, and phosphide semiconductors in general,have another advantage of being radiation resistant, which is ofparticular benefit in space applications.

Whether in the multiple junction or single junction PV device, aconventional characteristic of PV cells has been the use of a singlewindow layer on an emitter layer disposed on a base/substrate, which isshown for example in U.S. Pat. No. 5,322,573. Alternatively, the singlewindow layer is directly disposed on the base/substrate. In eitherinstance, the single window layer serves as a passivation layer wherebyminority carrier recombination is sought to be reduced at the frontsurface of the emitter layer (or base where there is no emitter layer).The reduction in surface recombination tends to increase cellefficiency.

Additionally, since the single window layer needs to be lighttransmissive, the single window layer has typically been relativelythin, i.e., less than about 100 nm thick and, for example, in the rangeof about 10 to 100 nm thick, as shown in U.S. Pat. No. 5,322,573. Infact, the past art has taught that the window layer should be as thin aspossible, such as in U.S. Pat. No. 5,322,573. However, some of thedisadvantages of the single window layer used in the past include thefact that minority carrier surface recombination can still occur to anextent that negatively impacts performance. Further, a relatively thinwindow layer may not present a satisfactory barrier to unwanteddiffusion of impurities, which can produce shunts or crystallinedefects. A thin window also provides less mechanical strength, smallersheet conductivity to supplement the emitter in reducing lateralconductivity losses, and may not allow sufficient freedom to minimizereflectance losses.

Similar to the conventional use of single layer window has been the useof a single layer back-surface field structure below the base/substrate,as shown in U.S. Pat. No. 5,800,630. The typical purpose of theback-surface field structure has been to serve as a passivation layerlike the single window layer described above. However, the disadvantagesof the single layer back-surface field structure include those mentionedabove with respect to a single window layer.

As can be seen, there is a need for an improved photovolaic cell,including one that can be incorporated into a multi-junction or singlejunction device. Also needed is an improved photovoltaic cell that is ofa heterojunction type and can include either a p-n or n-p junction.Another need is for a photovoltaic cell that has increased efficiencyand greater output voltage. A further need is for an improvedpassivation structure that can be used on a photovoltaic cell and, moregenerally, on a minority-carrier semiconductor device. Also needed is animproved passivation structure that can be used on a photovoltaic devicethat either contains or does not contain an emitter layer and/orback-surface field structure. A photovoltaic cell with a base made fromGe, GaAs, or GaInP is also needed with better efficiency.

SUMMARY OF THE INVENTION

The present invention is directed to an improved photovolatic cell thatcan improve efficiency in a multi-junction or single junction device.The improved photovoltaic cell is of a homojunction or a heterojunctiontype and can include either a p-n junction or n-p junction. Thepassivation structure of the present invention can not only be used on aphotovoltaic cell, but on minority-carrier semiconductor devices ingeneral. Further, the passivation structure of the present invention canbe used on a photovoltaic device that either contains or does notcontain an emitter layer and/or back-surface field structure. Amongothers, the base of the photovoltaic cell in the present invention canbe made of Ge, GaAs, or GaInP.

Specifically, the improved photovoltaic cell includes a base layer of afirst type of doping; a primary window layer having a second type ofdoping, with the primary window layer being disposed over the baselayer; and a secondary window layer having the second type of doping,with the secondary window layer being disposed over the primary windowlayer.

In another embodiment of present invention, the improved photovoltaiccell includes a base layer of a first type of doping; an emitter layerof a second type of doping, with the emitter layer being disposed overthe base layer; a primary window layer having a second type of doping,with the primary window layer being disposed over the emitter layer; anda secondary window layer having the second type of doping, with thesecondary window layer being disposed over the primary window layer.

In yet another embodiment of present invention, the improvedphotovoltaic cell has a multilayer back-surface field structure; a baselayer disposed over the back-surface field structure; and a primarywindow layer disposed over the base layer.

In a further embodiment, the photovoltaic cell of the present inventionincludes a base layer; and a primary window layer disposed over the baselayer, with the primary window layer having a thickness of at leastabout 1000 Angstroms.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a diagram of an n-p type photovoltaic cell having an emitteraccording to the present invention;

FIG. 1b is a diagram of a p-n type photovoltaic cell having an emitteraccording to the present invention;

FIG. 2a is a diagram of an n-p type photovoltaic cell without an emitteraccording to the present invention;

FIG. 2b is a diagram of a p-n type photovoltaic cell without an emitteraccording to the present invention;

FIG. 3a is a diagram of an n-p type photovoltaic cell having an emitterand multilayer back-surface field structure according to the presentinvention;

FIG. 3b is a diagram of a p-n type photovoltaic cell having an emitterand multilayer back-surface field structure according to the presentinvention;

FIG. 4 is a graph depicting the sum of the measured current densityobtained from measured external quantum efficiency for GaInP and GaAscomponent photovoltaic cells with a bilayer window on the Ga-As basecell as shown in FIG. 1, relative to that obtained for GaInP and GaAscomponent cells with a single layer window on the GaAs base cell;

FIG. 5 is a graph depicting internal quantum efficiency vs. wavelengthfor a photovoltaic cell having a bilayer window on a GaAs base componentcell, in accordance with FIG. 1, as well as a photovoltaic cell having asingle window layer;

FIG. 6 depicts a satellite on which the photovoltaic cell of the presentinvention is utilized as part of a solar panel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The improved photovoltaic cell of the present invention is describedbelow in various embodiments. In general, however, the photovoltaic cellof the present invention includes a window structure that acts as apassivation structure. More specifically, and according to oneembodiment of the present invention, the window or passivation structureis made of two layers. In another embodiment (not shown in thedrawings), the window or passivation structure is made of a singlelayer. The window structure receives solar energy and transmits it to anemitter layer and then to the base layer. Alternatively, the solarenergy is transmitted from the window structure directly to a base layersuch that the window structure also acts as an emitter layer.Optionally, the photovoltaic cell of the present invention can include amultilayer back-surface field structure that acts as a passivationstructure.

Furthermore, while the present invention may be particularly useful inthe context of spacecraft, such as in a solar panel 71 of a satellite 70(FIG. 6), other applications--both terrestrial and non-terrestrial--arecontemplated. Still further, even though the present invention isdescribed in the context of a photovoltaic cell, the invention is not solimited. Other contexts, such as heterojunction bipolar transistors,photodetectors, laser diodes, light emitting diodes, andminority-carrier semiconductor devices in general, are contemplated tobe within the scope of the present invention.

Given the breadth of contexts of the present invention, it can beappreciated by those skilled in the art that the different semiconductorlayers that comprise the minority-carrier semiconductor device of thepresent invention can be made by many well known processes in the art,such as molecular beam epitaxy (MBE), liquid phase epitaxy (LPE) andmetal-organic chemical vapor deposition (MOCVD). In accordance with suchknown methods, the specific materials comprising the semiconductorlayers may be altered and optimized to meet the requirements of theparticular context.

FIG. 1a depicts a photovoltaic cell 10 according to one embodiment ofpresent invention. In this embodiment, the cell 10 includes a window orpassivation structure 15 that can receive solar energy preferably afterthe energy has first passed through an antireflection layer or coating(not shown)--or another component cell in a multifunction solar cell(also not shown)--that is disposed on top of the window structure 15, asviewed from FIG. 1a. The antireflection layer is intended to minimizesurface reflections between the air and semiconductor layers of the cell10, thereby enabling more photons to enter the cell 10. Theantireflection layer can be made from well known materials in the art,such as Al₂ O₃, TiO₂, SiO₂ or Ta₂ O₅. The thickness of theantireflective coating can vary, but is typically between 30 nm to 150nm.

In FIG. 1a, the window structure 15 is of a multilayer type. Morespecifically, in this particular embodiment, the window structure 15 isa bilayer type and, therefore, includes two layers. However, the presentinvention contemplates that the window structure 15 can include morethan 2 layers. Nevertheless, for this embodiment, a secondary windowlayer 11 is disposed directly below the antireflective coating (notshown) and a primary window layer 12 is disposed directly below thesecondary window layer 11 and above an emitter layer 13 describedhereinafter. The secondary window layer 11 and primary window layer 12are made of semiconductor materials having a first type doping, which isfurther described below. Although not necessary, the windows 11,12 arepreferably lattice matched to one another.

The function of the window or passivation structure 15 is, in part, topassivate the emitter layer 13 which is made of a semiconductor materialhaving the first type of doping and disposed directly under the windowstructure 15. In other words, as photogeneration occurs in the emitterlayer 13, minority-carrier recombination (i.e., recombination ofelectrons and holes) tends to occur at a front surface (or interface) 16of the emitter layer 13 where the solar energy enters such layer 13. Butthe provision of the primary window layer 12 serves to minimize thesurface (or interface) recombination.

The ability of the primary window layer 12 to minimize surface (orinterface) recombination is due to it having a wider energy bandgap incomparison to the emitter layer 13. Still, since photogeneration canoccur in the primary window layer 12, minority-carrier recombination canalso occur at a front surface 17 of the primary window layer 12 thatinterfaces with the secondary window layer 11. Therefore, the secondarywindow layer 11 is provided to minimize the surface (or interface)recombination at the surface (or interface) 17 of minority carriers inthe primary window layer 12. This minimization is provided by thesecondary window 11 by having a wider energy bandgap than the primarywindow 12. With reduced minority-carrier recombination at the frontsurface 17 of the primary window 12, a greater amount of current densitycan be collected from the total current density that is photogeneratedin the primary window 12. Additionally, that same reduction minimizesthe loss of minority-carriers that can be injected from the emitterlayer 13 and into the primary window 12, thereby resulting in a highervoltage from the cell 10.

As can therefore be appreciated, additional window layers can beprovided over the two window layers 11,12 to additionally minimizesurface recombination that may occur at a front surface 18 of thesecondary window 11. Yet, there is a practical limitation to the totalnumber of windows since the added minimization does not increaselinearly.

Irrespective of the number of window layers, and for the purpose ofminimizing surface recombination, the difference in bandgap widthsbetween adjacent windows in the progression of window layers movingupwards towards the antireflection layer need only be appreciable enoughfor the minority carriers to "see" wider and wider bandgaps.Nevertheless, the greater the difference in bandgaps of adjacentwindows, the greater minimization of surface recombination, although theincreased minimization will not be linear. On the other hand, as thedifference in bandgap widths increase, then disadvantages can occur,such as difficulty in doping wide-bandgap materials. Accordingly, thedifference in bandgap widths between the secondary and primary windows11,12 should preferably range from about 30 meV to 1000 meV. Below about30 meV, then surface recombination is not significantly suppressed;while above about 1000 meV, the above disadvantages tend to result.

Another function of the secondary window layer 11 can be to blockunwanted diffusion of impurities such as dopants, or of point defects,into or out of the primary window layer 12. This is accomplished byvirtue of greater physical distance (thickness) and improved control ofstress (e.g., through the use of differing composition, doping, andthermal expansion coefficients in the layers) at primary and secondarywindow interfaces 17,18. In a contrary fashion, if such diffusion isdesired, the secondary window layer 11 may be used to enhance thediffusion by varying the parameters of the layer such as the onesdescribed above. The secondary window layer 11 may also serve as abetter template, in comparison to the primary window layer 12, forgrowing subsequent high quality crystalline layers.

Given the above operating parameters of the secondary window layer 11,it can be seen that the window layers 11,12 can be made of a wide rangeof materials. As examples, the secondary window layer 11 can be made ofGaInP, AlInP, AlGaInP, AlGaAs, GaInAsP or AlAs. With such materials forthe secondary window layer 11, the primary window layer 12 may berespectively made, for example, of GaAs, GaInP (or GaAs), GaInP (orGaAs), GaInP (or GaAs), AlGaInP (or GaInP and GaAs), GaInAsP, AlInAsP,AlGaAsP or AlGaAs (or AlGaInP, GaInP and GaAs).

For the n-p type device shown in FIG. 1a, the first type of doping inboth window layers 11,12 is n-type doping. The particular dopant usedcan vary according to well known methods in the art. For example, if thesecondary window layer 11 is made of AlInP and the primary window layer12 is made of GaInP, then the dopant in the secondary window 11 can besilicon and the dopant in the primary window 12 can be tellurium.Consequently, the dopant concentration in the primary and secondarywindows 11,12 can be about 1 E17 to 1 E19 to (and preferably about 5 E17to 5 E18).

Just as the types of materials can vary for the window layers 11,12,their thicknesses can also vary. Typically, however, the secondarywindow 11, in this embodiment, may range from about 100 Angstroms to2000 Angstroms, although preferably about 250 Angstroms to 500Angstroms. With such a thickness range for the secondary window 11, theprimary window 12 may have a thickness that ranges from about 250Angstroms to 5000 Angstroms, and preferably about 1000 Angstroms to 3000Angstroms.

As shown in FIG. 1a, the emitter layer 13 is where some photogenerationand current generation occur in the cell 10, with most occurring in thebase layer. The material comprising the emitter layer 13 is dependentupon the materials used for the window layers 11,12, as well as the baselayer 14 described below. With the above examples for the window layers11, 12, the emitter 13 can be made, for example, of InGaAsN, GaInAs,AlGaAs, GaInAsP, AlGaInP, and preferably GaAs or GaInP. The optimalthickness of the emitter layer 13 will also vary with the thickness ofthe window layers 11,12. Using the above thickness ranges for the windowlayers 11,12, the emitter layer 13 thickness can range from about 1000Angstroms to 3000 Angstroms, and preferably from about 1500 Angstroms to2500 Angstroms. As indicated in FIG. 1a, the emitter layer 13 containsthe first type doping. Some examples of useful dopants in the emitter 13include silicon, selenium, tellurium, tin (n-type dopants), althoughpreferably tellurium. The dopant concentration in the emitter 13 cantypically vary between about 0.9E18 to 5E18, and preferably betweenabout 1E18 to 3E18.

Directly below the emitter layer 13 for the embodiment shown in FIG. 1ais a base or substrate layer 14 that provides structural integrity tothe cell 10. Because this embodiment is an n-p type device, the base 14and the substrate are made of materials having a second typedoping--namely, p-type doping. Useful dopants include carbon, zinc,magnesium (p-type), even though zinc is preferred. Thereby, an n-pjunction is formed at the junction (either homojunction orheterojunction) of the emitter layer 13 and the base layer 14. Acrosssuch junction, a voltage difference at the junction can be created dueto the excess charge carriers photogenerated by the solar energyimpingement. From the voltage difference, an output current is createdthat is carried out of the cell 10 via the base layer 14 and to anexternal termination (not shown).

To achieve its functions, the base layer 14 preferably has a bandgapwidth substantially the same as the emitter layer 13. With the need forequivalent bandgap widths, various materials can be used for the base14, such as GaAs, GaInP, InGaAsN, AlGaAs, AlGaInP, GaInAs and GaInAs.The thickness of the base layer 14 can vary. But with the abovethickness ranges for the emitter layer 13 and window structure 15, thebase layer 14 thickness can typically range from about 0.25 to 5micrometers, and preferably from about 0.3 to 4 micrometers.

FIG. 1b depicts another embodiment of the present invention. Theembodiment is identical to that shown in FIG. 1a, except that thephotovoltaic cell 20 is of a p-n type. Thus, the cell 20 includes abilayer window or passivation structure 25 having a secondary windowlayer 21 and a primary window layer 22, both of which are made of ap-type doped material. An emitter layer 23 is disposed below thepassivation structure 25 and made of a p-type doped material. The p-njunction is formed by having the base 24 made of an n-type dopedmaterial.

FIG. 2a depicts yet another embodiment of an n-p type device accordingto the present invention. Therein, a photovoltaic cell 30 has the sameconstruction of the cell 10 shown in FIG. 1a, except that no emitterlayer is provided. As shown in FIG. 2a, the photovoltaic cell 30includes a multilayer window or passivation structure 34 having asecondary window layer 31 and a primary layer window layer 32. With theomission of an emitter layer, the primary window layer acts 32 as bothan emitter and a window. Both windows 31,32 are made of materials thathave the same doping type--namely, n-type doping. Directly below thepassivation structure 34, as viewed from FIG. 2a, is a base layer orsubstrate 33. Most of the photogeneration occurs in the base layer 33,as opposed to a separate emitter layer of the same bandgap.Consequently, a front surface 35 of the base 33 can be whereminority-carrier recombination occurs, but which is minimized by thewindow structure 34.

For the embodiment shown in FIG. 2a, the secondary window layer 31 andprimary window layer 32 can be made of the same materials and dopants asthe windows 11,12 for the embodiment in FIG. 1a. Similarly, thethicknesses of the windows 31,32 may be of the same thicknesses for thewindows 11,12. The thickness, materials, dopants and dopantconcentrations of the base 33 are similar to the embodiment shown inFIG. 1a.

FIG. 2b shows the same embodiment of the invention shown in FIG. 2a,except the former is for a p-n type device. As such, a photovoltaic cell40 has a window or passivation structure 44 made of p-type dopedmaterials in a secondary window layer 41 and primary window layer 42.With the passivation structure 44 being p-type, a base 43 is n-type.

FIG. 3a shows another embodiment of the present invention which isidentical to that shown in FIG. 1a, except with the addition of amultilayer back-surface field (BSF) structure 58. With a photovoltaiccell 50 being of an n-p type, a window or passivation structure 57includes n-doped material for both a secondary window layer 51 and aprimary window layer 52. The difference in bandgap widths is similar tothat of the passivation structure 15 of the FIG. 1a so thatminority-carrier recombination at a front surface 59 of an emitter layer53 can be reduced. The semiconductor materials, dopants, concentrations,and the thicknesses of the layers 51,52 can be similar to the layers11,12 shown in FIG. 1a. In FIG. 3a, the emitter layer 53 is made of ann-type material and of a thickness similar to the emitter layer 13 shownin FIG. 1a. Likewise, a base layer 54 in FIG. 3a is made of a p-typematerial and of a thickness similar to the base layer 14 shown in FIG.1a.

The BSF structure 58, as shown in FIG. 3a, is a bilayer type andincludes a primary back-surface field layer 55 and secondaryback-surface field layer 56. The BSF structure 58 provides a passivationfunction similar to the window structure 57, except that the BSFstructure 58 minimizes minority-carrier surface recombination at a backsurface 60a of the base 54, as well as a back surface 60b of thesecondary BSF layer 56. Accordingly, the bandgap for the secondary BSFlayer 56 is greater than that of the primary BSF layer 55. The BSFstructure 58 can have more than two layers, as can the window structure57. The BSF layers 55,56 are made of p-type doped materials. Asexamples, the primary BSF layer 55 can be made of GaAs, GaInP (or GaAs),GaInP (or GaAs), GaInP (or GaAs), AlGaInP (or GaInP and GaAs), GaInAsP,AlInAsP, AlGaAsP or AlGaAs (or AlGaInP, GaInP and GaAs) while thesecondary BSF layer 56 can be made of of GaInP, AlInP, AlGaInP, AlGaAs,GaInAsP or AlAs. The dopants for the BSF layers 55,56 can includecarbon, magnesium and zinc, although zinc is preferred. The dopantconcentrations for both layers 55,56 can range from about 1E18 to 1E20and preferably about 5 E18 to 2 E19. The thicknesses of the BSF layers55,56 can vary. However, with the above thicknesses for the other layersin the cell 50, the BSF layer 55 can range from about 100 Angstroms to4000 Angstroms, and the BSF layer 56 can also range from about 100Angstroms to 4000 Angstroms. Preferably, the thicknesses range fromabout 500 Angstroms to 2000 Angstroms, and 200 Angstroms to 500Angstroms, respectively.

FIG. 3b shows the same embodiment of the invention shown in FIG. 3a,except the former is for a p-n type device. As such, a photovoltaic cell69 has a window or passivation structure 67 made of p-type dopedmaterials in a secondary window layer 61 and primary window layer 62. Anemitter layer 63 is also p-type doped. With the passivation structure 67being p-type, a base 64 is n-type. A multilayer back-surface field (BSF)structure 68 includes a primary back-surface layer 65 and a secondaryback-surface layer 66, both of which are n-type doped.

In yet another embodiment of the present invention, the photovoltaiccells depicted in FIGS. 1a through 3b can be constructed with apassivation structure having only a single "thick" window layer. Thereference to "thick" is intended to distinguish the so-called "thin"single window layers used in the past. In particular, the "thick" singlewindow layer of the present invention is characterized by a thickness ofabout 1000 Angstroms and greater. Preferably, the thickness ranges fromabout 1500 Angstroms to 3000 Angstroms. Below a thickness of about 1000Angstroms, the beneficial effects of the window layer are diminished.Above a thickness of about 4000 Angstroms, light absorption in thewindow becomes excessive, and more time and materials than necessary areused in MOVPE growth.

As with the mutilayer window structure of the present invention, thesingle "thick" window layer can serve to block unwanted diffusion ofimpurities and/or point defects. It can also be used to enhance suchdiffusion. Accordingly, the single "thick" window of the presentinvention can be made from materials such as GaInP, AlInP,AlGaAs,AlInAsN and all previously mentioned primary and secondary window layermaterials. The single window can include dopants such as silicon,selenium, tellurium and tin, but preferably silicon or tellurium. Thedopant concentrations typically range from about 1E17 to 1E19, while 5E17 to 5 E18 is preferred.

As can be appreciated by those skilled in the art, the photovoltaic cellof the present invention is not limited to use as a single junctioncell. It can be incorporated into a multijunction photovoltaic cell.Further, the window or passivation structure of the present inventioncan be used in heterojunction semiconductor devices in general.

EXAMPLES

In FIG. 4, the data were measured on cells from several experiments totest the advantages of the bilayer window according to the presentinvention. The `thick` GaInP middle-cell window (MCW) is a single-layerwindow of GaInP with a thickness of about 1500 to 3000 Angstroms. The`thick/thin` GaInP/AlInP MCW is a bilayer window with a primary windowlayer of GaInP with a 1500 to 2500 Angstrom thickness, and a secondarywindow layer of AlInP with a 250 to 500 Angstrom thickness. In FIG. 5,the data was obtained using standard solar cell testing methods on cellsfrom the above experiments.

As can be appreciated by those skilled in the art, the present inventionprovides improved efficiency in a multi-junction or single junctiondevice. The improved photovoltaic cell is of a homojunction or aheterojunction type and can include either a p-n junction or n-pjunction. Further, the passivation structure of the present inventioncan be used on a photovoltaic device that either contains or does notcontain an emitter layer and/or back-surface field structure.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

What is claimed is:
 1. A photovoltaic cell, comprising:a multilayerback-surface field structure in which one layer of said structurepassivates another layer of said structure; a base layer disposed oversaid back-surface field structure; a primary window layer disposed oversaid base layer and characterized by a primary energy bandgap; and asecondary window layer disposed over said primary window layer, saidsecondary window layer being characterized by a secondary energy bandgapthat is greater than said primary energy bandgap in order to passivatesaid primary window layer and thereby increase collection ofphotogenerated current from said primary window layer.
 2. Thephotovoltaic cell of claim 1, further comprising an emitter layerdisposed intermediate said base layer and primary window layer.
 3. Thephotovoltaic cell of claim 2, further comprising one of a p-n junctionand an n-p junction located at a heterojunction formed between said baselayer and emitter layer.
 4. The photovoltaic cell of claim 1, furthercomprising one of a p-n junction and an n-p junction located at aheterojunction formed between said base layer and said primary windowlayer.
 5. The photovoltaic cell of claim 1, wherein said multilayerback-field passivation structure comprises a primary BSF layer and asecondary BSF layer, said secondary BSF layer being characterized by asecondary bandgap greater than a primary bandgap associated with saidprimary BSF layer so that the secondary BSF layer passivates the primaryBSF layer.